Re: a crazy idea?

Date: 2018-05-25 12:52 pm (UTC)
From: (Anonymous)
Well the certificate is public data, so assuming an attacker has it and you aren't tying your secrets to a measurement of your GRUB/configuration, then they could just swap in their own GRUB module or configuration and extend the PCR themselves with the certificate. Also, the certificate would need to be measured as part of this anyways, even before verification checks out, if that cert comprises your "trust store", otherwise there is the swap out the cert attack.

At that point, if you're already measuring the GRUB software/configuration/cert, doesn't seem like you would need to extend with anything else. If your GRUB software and configuration is getting measured, and the logic declares boot this kernel on correct sig verification or exit on failure, then tying your secrets to those measurements and later receiving them from the TPM means you've already verified that you booted a signed kernel, and what you're measuring is still less volatile.
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Matthew Garrett

About Matthew

Power management, mobile and firmware developer on Linux. Security developer at Google. Ex-biologist. @mjg59 on Twitter. Content here should not be interpreted as the opinion of my employer.

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